Above: Image by Raver Duane, U.S. Fish and Wildlife Service, Wikimedia Commons

The feminization of fish is a problem that has been observed around the globe in freshwater and marine systems. What does this mean and why is it happening?

Feminization of fish occurs when long-lasting changes take place in the reproductive organs of the male fish. These changes can result in delayed sexual maturation, a lower sperm count, early-stage eggs in the testes and the production of vitellogenin, a protein associated with egg production in the female fish when exposed to environmental contaminants.

The presence of various pharmaceuticals, pesticides and consumer products in the environment can lead to feminization of the male fish. Human beings consume a wide range of drugs, which are excreted into wastewater. Drugs can also enter wastewater through improper disposal of medications by flushing them down the toilet or pouring them in a drain. Two types of drugs often found in wastewater are birth control pills and hormone replacement therapy drugs.

Did you know? Birth control pills contain ethinyl estradiol, a synthetic estrogen hormone, and hormone replacement therapy drugs contain equine estrogens, which are natural, but usually made synthetically (and they are horse hormones)!

When male fish in lakes and streams are exposed to the estrogen in these drugs, their reproductive organs start to deform and function like female reproductive organs.

Did you know? Estrogen-induced feminization of fish results in more female than male fish in a population and the presence of fish with both male and female organs.

The phenomenon of feminization of fish has been widely researched. In a study conducted by Karen Kidd, for example, a female sex hormone, 17α-ethinylestradiol, was introduced into a lake predominately containing a fish called "fathead minnow". The following autumn, Kidd noticed a delay in the development of the sperm cell in the male fathead minnow. A year later, the male fathead minnows were producing eggs instead of sperm and had stopped reproducing. A significant decline in the fish was observed and the entire population was wiped out within three years. Once the fathead minnow disappeared, there was a 30 per cent decline in the population of lake trout, a species that feeds on smaller fish like the fathead minnow. The disappearance of a key member of the trout’s food chain led to this decrease in population. This study showed that trace amounts of estradiol can wipe out an entire population of minnows.

Feminization of fish is a serious environmental concern that researchers continue to study in the hopes of discovering how to deal with the issue.

Learn More!

“Chemicals are Likely Cause of Feminization of Fish Present in Two Rivers in Alberta, Canada, ResearchersFind”, Science Daily, July 30, 2010.

Chemical and Engineering News: Feminization of Fish


Article first published February 4, 2011.

Kiran Anwar

I have recently completed my Masters in Environmental Science from the University of Toronto. I am currently working as a biologist for Health Canada where I am conducting Human Health Risk Assessment on new substances that are entering the Canadian consumer market. I am really interested in writing for CurioCity.

Comments are closed.